This invention relates to solar blind optical filters and, more particularly, to dye compounds used in such filters.
Many important phenomena produce ultraviolet radiation including fires, rocket and jet engine exhausts, electrical discharges on high tension wires, lightning, and the plasma surrounding an object that is entering the earth's atmosphere at a high velocity. Photons of ultraviolet radiation are more energetic than photons of visible light and are therefore easier to detect. There are many sensors that detect UV radiation including photodiodes, photomultipliers, charge-coupled device (CCD) arrays, and other light sensors familiar to those practiced in the art of optical detection. The sensors typically respond to light over a range or spectral band of wavelengths.
The detection of an ultraviolet emission source such as a fire or rocket plume during the day is complicated by ultraviolet light that is emitted by the sun and only partly absorbed by the atmosphere. FIG. 1 shows the solar radiation or actinic flux, A(.lambda.), that is observed under typical conditions at sea level. If the response of the detector is S(.lambda.), then this actinic flux B will give a uniform background signal that is the integral of these two functions over wavelength according to the following equation: EQU B=.intg.S(.lambda.)A(.lambda.)d.lambda. (1)
The ultraviolet signal, T from the target process is a similar integral where A(.lambda.) is now replaced by the wavelength-dependent target emission E(.lambda.). Equation (1) then becomes: EQU T=.intg.S(.lambda.)E(.lambda.)d.lambda.. (2)
A useful figure of merit for a detection of a target process is the ratio of target signal to background, T/B. Sensors that provide high values for the ratio T/B are known as solar blind detectors. Qualitatively, one sees that this ratio is maximized when the actinic flux is completely excluded from the sensor since the value of B approaches zero.
As is seen in FIG. 1, the actinic flux increases very rapidly as .lambda. increases beyond 285 nm. Although the sun emits a large quantity of radiation at these wavelengths, such radiation is efficiently absorbed by the atmosphere, specifically by O.sub.3 in the stratosphere.
Current solar blind filters include dielectric stack filters such as those manufactured by Corion of Holliston, Mass. or Plummer Precision Optics of Boston, Mass. Dielectric stack filters can have a very rapid change of filter transmission with wavelength, however, the range of wavelengths which is transmitted varies rapidly with the angle of incidence and polarization of the incoming light. These constraints reduce the figure of merit T/B for many applications. Furthermore, the peak transmission of these dielectric stacks is typically less than 30%, so that T is less than ideal for stack filters.
Another known filter material uses absorbing dye materials such as those sold by Ofil, Ltd. of Israel. Dye molecules have the advantage that their optical transmission is independent of the incident angle and polarization of solar radiation. However, dye materials that absorb well at around 300 nm but then rapidly become transparent as the energy of the photon increases are rare. This is a fundamental consequence of the quantum electronic structure of materials, familiar to those practiced in the art of physical chemistry. The design issue for dyes is therefore to minimize transmission of background radiation B while retaining enough target radiation T (see equation (2)) to detect the target signal.
The actinic flux is computed from the sum of the direct, attenuated solar beam plus angularly integrated scattered radiation using the U.S. Standard Atmosphere (1976) and a solar zenith angle of 30.degree.. The variation of actinic flux with season, zenith angle, weather conditions, and latitude is familiar to those practiced in the art of aeronomy.